Isolation of Fc receptor from detergent-solubilized, radiolabelled lymphoid cells is being attempt. The affinity of Fc receptor for antigen-antibody complexes is being exploited as a means to selectively bind radiolabelled Fc receptor. Design of rigorous controls is being carried out. SDS polyacrylamide gel electrophoresis is used to analyze materials which are bound. The radiolabelled cell components which bind to antigen-antibody complexes do not appear to be similar to Ia antigens in molecular weight, although anti-Ia antisera are capable of blocking binding of fluoresceinated IgG aggregates to Fc receptor. The bound components do appear to be similar in molecular weight to molecules precipitated by another, relatively uncharacterized, alloantiserum which also blocks binding of fluoresceinated IgG aggregates to Fc receptor. Further correlation between these entities is being carried out.